-LRB- CNN -RRB- -- Climate scientists are getting desperate . After years of enduring politically motivated attacks , they are still scrambling to defend their findings .

In a letter last month to Science , 255 of the nation 's most prominent scientists -- all members of the National Academy of Sciences , including 11 Nobel laureates -- repeat conclusions that by now should be entirely familiar , even tedious , to anyone who has followed the science at all . Our planet is warming because of increased greenhouse gases in the atmosphere .

Burning fossil fuels and deforestation have caused most of the increase . Climate change is a threat to coastal communities , food and water supplies , and ecosystems . And contrary to some recent claims , it is not too late to do something about it . The scientists conclude , in obvious exasperation , that one `` snowy winter in Washington does not alter '' these basic facts .

The immediate trigger for the letter Virginia State Attorney General Kenneth T. Cuccinelli 's recent demand that climate scientist Michael Mann , director of the Earth System Science Center at Penn State University , produce a large volume of paperwork related to his scientific research and communications with fellow scientists during his years at University of Virginia from 1999 to 2005 . `` McCarthyite '' is how the scientists characterize Cuccinelli 's action , filed under the state 's Fraud Against Taxpayers Act .

Many have dismissed Cuccinelli as silly , particularly in light of his efforts to impose Christian modesty on the state seal of Virginia , and his actions could be shrugged off if they were isolated . But they are the latest example of a persistent , and worrisome , trend : attacking scientists who have done key work that demonstrates the human impact on climate .

Mann is the principal author of the `` hockey stick curve '' -- a graph that boldly illustrates the dramatic rise in average global temperatures over the past 50 years , a rise unlike anything in the previous 1,000 years . Because the hockey stick curve is visually compelling , it 's been reproduced many times , and in 2001 the Intergovernmental Panel on Climate Change featured it in their third assessment report . It 's also made Mann a target for those who do n't want to admit that global warming is a real problem .

Mann testified about his work in the U.S. Congress in 2003 , in hearings sponsored by Sen. James M. Inhofe of Oklahoma , who has called global warming a `` hoax . '' Two years later , Texas Rep. Joseph Barton demanded that Mann supply copious documentation related to his testimony , based on what Barton , who has no scientific training , alleged were `` methodological flaws and data errors '' in his work .

Peter Gleick , the lead author of the Science letter , was threatened with a lawsuit by contrarian Patrick J. Michaels in 2003 . Michaels has long denied the scientific evidence of human-caused warming , and Gleick said those views flew in the face of overwhelming scientific evidence and likened them to believing in a flat Earth .

In 1995 , a group called Global Climate Coalition , composed of the American Petroleum Institute and a host of fossil-fuel dependent corporations , attacked Benjamin Santer , a scientist at the Lawrence Livermore National Laboratory who had done key work demonstrating the human `` fingerprint '' on climate . Without any substantive evidence , the group accused Santer of committing `` scientific cleansing '' of the IPCC 's Second Assessment Report by removing mention of uncertainties to make global warming appear more certain than it was .

A handful of physicists attached to another think-tank , the George C. Marshall Institute , splashed the accusation onto the pages of the Wall Street Journal , ensuring millions of Americans saw them . Santer was later vindicated -- all the co-authors testified he had made no unauthorized changes . Yet the impression of wrong-doing remained .

Perhaps no scientific name is more associated with global warming than Roger Revelle , mentor to Al Gore , who first warned of the risks of human-caused climate change in the 1950s . In 1992 , a paper was released that claimed Revelle had changed his mind about global warming and no longer believed it was a problem . The claim was repeated many times in the mass media , including The New Republic and The Washington Post .

Revelle was elderly and seriously ill at the time , and died before he had a chance to respond . His graduate student , Justin Lancaster , tried to set the record straight by publicly disputing the claim . He was soon sued by the author of the claim , and lacking funds to defend himself , was forced to settle out of court , leaving his personal and professional life in tatters .

But it is n't just climate scientists who have been vilified , personally attacked , and threatened with lawsuits and congressional investigations . Since the early 1990s , there has been a sustained history of attempts to undermine any science that suggested that contemporary industrial society might be doing irreparable harm to human health and the natural environment .

This included the science that demonstrated the harms of DDT , the dangers to children of second-hand smoke , the causes of acid rain , and the reality of the ozone hole . Often the same people were involved in several or even all of these attacks . The common feature in all these cases was a link to think tanks promoting free markets and opposing government regulations .

One does n't have to be a conspiracy theorist to see the pattern : People are loath to admit that our free market system has created problems that the free market has proved ineffectual to solve .

Nicolas Stern , former chief economist at the World Bank , has called global warming `` the greatest and widest-ranging market failure ever seen . '' Deaths from tobacco smoking , environmental destruction from DDT and other pesticides , acid rain and the ozone hole were market failures , too . Insufficiently regulated market capitalism created these problems , and it took government interventions -- and in some cases , international agreements -- to fix them .

If you tried to argue that global warming is not a market failure , you 'd just look silly . So extremist defenders of the free market have found themselves an easier target . Science is arcane and scientists are frankly often incomprehensible . But the important point for us to understand is this : Scientists have done nothing wrong . On the contrary , they 've been repeatedly vindicated in their work on environmental change .

The real crime is not to be found in some pile of obscure scientific documents or the e-mails of harassed scientists . The real crime is that our best science is being undermined by ideologues , confusing us about some of the most important issues of our time .

The opinions expressed in this commentary are solely those of the authors .

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Oreskes , Conway : 255 scientists wrote letter to reiterate global warming findings

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Authors : Letter came after attack on scientist whose work shows human impact on climate

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They say attacks similar to those aimed at science on DDT , second-hand smoke , acid rain

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Common to all , they say , is link to think tanks promoting free markets , opposing regulations